The present invention relates to wideband imaging. In particular, axial whitening is provided for detail and contrast resolution in medical diagnostic ultrasound imaging.
Transmit electrics, the transducer, propagation, reflection and receive circuitry reduce or alter the bandwidth of transmitted information. Axial resolution is reduced as the bandwidth of the signal decreases. To improve bandwidth, transmit signals with a maximum or greater bandwidth are transmitted. For example, a transmit waveform with a low-pass filtered envelope is generated with a single center frequency or with a chirped frequency range. The spectral magnitude is constant over a range of the imaging frequency band. The white or flat portion of the spectral magnitude may be as wide as allowable by the transmit circuitry. The white transmit spectral amplitude more likely provides wideband or whitened receive information for imaging. However, the filtering or response of various circuits, tissue and targets may still reduce the bandwidth.
Axial whitening is provided on the receive signal to increase bandwidth. U.S. Pat. Nos. 4,881,549, 4,470,303, 6,358,209 and 6,110,115 disclose techniques for whitening the receive signal. Downstream of the acoustic interrogation of a target field, the signal is selectively filtered so that non-equal spectral magnitude within an imaging frequency band is adjusted to provide a white or substantially flat spectral magnitude. Pre- or post-detected data may be whitened, but the bandwidth is still limited by the response or transfer function of the transmit circuitry, propagation, the target and receive circuitry. For whitening pre-detected data, a base band filter is adapted to suppress DC components or accentuate out-of-band components. For post-detected data, a video filter accentuates out-of-band components.
The spectral content of the transmit waveform may be altered for various reasons. For example, U.S. Pat. No. 6,312,379 discloses predistortion of a transmit waveform for elimination of transmitted energy in a second harmonic of a fundamental transmit frequency band. To better isolate information at the second harmonic frequencies generated by propagation or reflection rather than by transmitted energy, energy transmitted at the second harmonic is counteracted by predistortion. An inverse second harmonic component is added to the transmit waveform so that transmit circuitry and transducer generated second harmonics are counteracted or removed. By removing energy at the second harmonic frequency band in a transmit pulse generated by the transducer, clutter due to the transmit circuitry and transducer for second harmonic imaging is removed or reduced. As another example, U.S. application Ser. No. 10/441,325 discloses using bi-modal or dual frequency cascaded transmit pulses, resulting in spectral components at two different frequencies for receiving information at an intermodulation frequency bands that may or may not overlap with one of the two transmitted frequency bands. By cascading pulses of different frequencies, spectral phase varies greatly due to the distinct frequency components occurring in distinct separate times. The spectral phase can be flat for frequencies corresponding to a first pulse but then will ramp significantly for frequencies corresponding to the second pulse, i.e. the phase is in non-linear. The use of cascaded pulses also may result in less detail resolution.